The Apolipoprotein E ε4 Allele-Dependent Relationship Between Serum Lipid Levels and Cognitive Function: A Population-Based Cross-sectional Study

Roles of peripheral lipoproteins and cholesteryl ester transfer protein in the vascular contributions to cognitive impairment and dementia

This narrative review focuses on the role of cholesteryl ester transfer protein (CETP) and peripheral lipoproteins in the vascular contributions to cognitive impairment and dementia (VCID). Humans have a peripheral lipoprotein profile where low-density lipoproteins (LDL) represent the dominant lipoprotein fraction and high-density lipoproteins (HDL) represent a minor lipoprotein fraction. Elevated LDL-cholesterol (LDL-C) levels are well-established to cause cardiovascular disease and several LDL-C-lowering therapies are clinically available to manage this vascular risk factor. The efficacy of LDL-C-lowering therapies to reduce risk of all-cause dementia and AD is now important to address as recent studies demonstrate a role for LDL in Alzheimer's Disease (AD) as well as in all-cause dementia. The LDL:HDL ratio in humans is set mainly by CETP activity, which exchanges cholesteryl esters for triglycerides across lipoprotein fractions to raise LDL and lower HDL as CETP activity increases. Genetic and pharmacological studies support the hypothesis that CETP inhibition reduces cardiovascular risk by lowering LDL, which, by extension, may also lower VCID. Unlike humans, wild-type mice do not express catalytically active CETP and have HDL as their major lipoprotein fraction. As HDL has potent beneficial effects on endothelial cells, the naturally high HDL levels in mice protect them from vascular disorders, likely including VCID. Genetic restoration of CETP expression in mice to generate a more human-like lipid profile may increase the relevance of murine models for VCID studies. The therapeutic potential of existing and emerging LDL-lowering therapies for VCID will be discussed. Figure Legend. Cholesteryl Ester Transfer Protein in Alzheimer's Disease. CETP is mainly produced by the liver, and exchanges cholesteryl esters for triglycerides across lipoprotein fractions to raise circulating LDL and lower HDL as CETP activity increases. Low CETP activity is associated with better cardiovascular health, due to decreased LDL and increased HDL, which may also improve brain health. Although most peripheral lipoproteins cannot enter the brain parenchyma due to the BBB, it is increasingly appreciated that direct access to the vascular endothelium may enable peripheral lipoproteins to have indirect effects on brain health. Thus, lipoproteins may affect the cerebrovasculature from both sides of the BBB. Recent studies show an association between elevated plasma LDL, a well-known cardiovascular risk factor, and a higher risk of AD, and considerable evidence suggests that high HDL levels are associated with reduced CAA and lower neuroinflammation. Considering the potential detrimental role of LDL in AD and the importance of HDL's beneficial effects on endothelial cells, high CETP activity may lead to compromised BBB integrity, increased CAA deposits and greater neuroinflammation. Abbreviations: CETP - cholesteryl transfer ester protein; LDL - low-density lipoproteins; HDL - high-density lipoproteins; BBB - blood-brain barrier; CAA - cerebral amyloid angiopathy, SMC - smooth muscle cells, PVM - perivascular macrophages, RBC - red blood cells.

Apolipoprotein E knockout may affect cognitive function in D-galactose-induced aging mice through the gut microbiota–brain axis

The gut microbiota plays an important role in central nervous system (CNS) disorders. Apolipoprotein E (ApoE) can affect the composition of the gut microbiota and is closely related to the CNS. However, the mechanism by which ApoE affects cognitive dysfunction through the gut microbiota–brain axis has thus far not been investigated. In this study, we used wild-type mice and ApoE knockout (ApoE^–/–) mice to replicate the aging model and examined the effects of ApoE deletion on cognitive function, hippocampal ultrastructure, synaptophysin (SYP) and postsynaptic density 95 (PSD-95) in aging mice. We also explored whether ApoE deletion affects the gut microbiota and the metabolite profile of the hippocampus in aging mice and finally examined the effect of ApoE deletion on lipids and oxidative stress in aging mice. The results showed that the deletion of ApoE aggravated cognitive dysfunction, hippocampal synaptic ultrastructural damage and dysregulation of SYP and PSD-95 expression in aging mice. Furthermore, ApoE deletion reduced gut microbial makeup in aging mice. Further studies showed that ApoE deletion altered the hippocampal metabolic profile and aggravated dyslipidemia and oxidative stress in aging mice. In brief, our findings suggest that loss of ApoE alters the composition of the gut microbiota, which in turn may affect cognitive function in aging mice through the gut microbiota–brain axis.

Circulating folate concentrations and the risk of mild cognitive impairment: a prospective study on the older Chinese population without folic acid fortification

BACKGROUND

The longitudinal association between serum folate concentrations and the risk of cognitive impairment remains unclear in populations with low folate levels. We examined the association between serum folate concentrations and mild cognitive impairment (MCI) in older adults in China, where mandatory fortification of foods with folic acid was not implemented. We further explored if homocysteine (Hcy) and leukocyte telomere length (LTL) mediate the association between serum folate and MCI.

METHODS

We performed a longitudinal analysis of 3974 participants aged ≥ 60 years from the Tianjin Elderly Nutrition and Cognition (TENC) cohort study. The associations between serum folate level and the risk of cognitive impairment overall and stratified by apolipoprotein E (APOE) ε4 genotypes were evaluated using multivariable Cox proportional hazards models. The mediating effects of Hcy and LTL on the folate-MCI association were explored via a path analysis approach.

RESULTS

Within a 3-year follow-up, we documented 560 incident MCI cases. After multivariable adjustment, higher serum folate concentrations were associated with lower incidence of MCI, with hazard ratios (95% confidence interval) across quartiles of folate (from lowest to highest concentrations) of 1.00 (reference), 0.66 (0.52, 0.83), 0.57 (0.45, 0.73), 0.66 (0.52, 0.84), respectively (P for trend < 0.001). In mediation analyses, the status of serum folate deficiency and MCI were correlated via two intermediary pathways, Hcy and Hcy-telomere (P < 0.05).

CONCLUSIONS

Lower folate concentrations, independently of APOE genotype, were associated with increased risk of MCI among elderly Chinese people, a population with relatively low folate intake. Our data were compatible with the mediation hypothesis that the association between folate status and MCI was mediated by Hcy and LTL.

Genetic Regulatory Networks of Apolipoproteins and Associated Medical Risks

Apolipoproteins (APO proteins) are the lipoprotein family proteins that play key roles in transporting lipoproteins all over the body. There are nearly more than twenty members reported in the APO protein family, among which the A, B, C, E, and L play major roles in contributing genetic risks to several disorders. Among these genetic risks, the single nucleotide polymorphisms (SNPs), involving the variation of single nucleotide base pairs, and their contributing polymorphisms play crucial roles in the apolipoprotein family and its concordant disease heterogeneity that have predominantly recurred through the years. In this review, we have contributed a handful of information on such genetic polymorphisms that include APOE, ApoA1/B ratio, and A1/C3/A4/A5 gene cluster-based population genetic studies carried throughout the world, to elaborately discuss the effects of various genetic polymorphisms in imparting various medical conditions, such as obesity, cardiovascular, stroke, Alzheimer's disease, diabetes, vascular complications, and other associated risks.

Association between Apoϵ4 allele and cardiometabolic and social risk factors with cognitive impairment in elderly population from Bogota

Being an ϵ4 carrier in the Apoϵ gene has been suggested as a modifying factor for the interaction between cardio-metabolic, social risk factors, and the development of cognitive impairment.

Blood Triglyceride and High-Density Lipoprotein Levels Are Associated with Plasma Amyloid-β Transport: A Population-Based Cross-Sectional Study

BACKGROUND

Studies have found that blood lipids are associated with plasma amyloid-β (Aβ) levels, but the underlying mechanism is still unclear. Two Aβ transporters, soluble form of low-density lipoprotein receptor related protein-1 (sLRP1) and soluble receptor of advanced glycation end products (sRAGE), are crucial in peripheral Aβ transport.

OBJECTIVE

The aim was to investigate the effects of lipids on the relationships between plasma Aβ and transporter levels.

METHODS

This study included 1,436 adults aged 40 to 88 years old. Blood Aβ, sLRP1, sRAGE, and lipid levels were measured. Univariate and multivariate analyses were used to analyze the relationships between lipids and plasma Aβ, sLRP1, and sRAGE.

RESULTS

After adjusting for all possible covariates, high-density lipoprotein (HDL-c) was positively associated with plasma Aβ42 and sRAGE (β= 6.158, p = 0.049; β= 121.156, p < 0.001, respectively), while triglyceride (TG) was negatively associated with plasma Aβ40, Aβ42, and sRAGE (β= -48.389, p = 0.017; β= -11.142, p = 0.020; β= -147.937, p = 0.003, respectively). Additionally, positive correlations were found between plasma Aβ and sRAGE in the normal TG (Aβ40: β= 0.034, p = 0.005; Aβ42: β= 0.010, p = 0.001) and HDL-c groups (Aβ40: β= 0.023, p = 0.033; Aβ42: β= 0.008, p = 0.002) but not in the high TG and low HDL-c groups.

CONCLUSION

Abnormal levels of TG and HDL-c are associated with decreased Aβ and sRAGE levels. Positive correlations between plasma Aβ and sRAGE were only found in the normal TG and HDL-c groups but not in the high TG and low HDL-c groups. These results indicated that dyslipidemia contributing to plasma Aβ levels might also be involved in peripheral Aβ clearance.

Apolipoprotein E Induced Cognitive Dysfunction: Mediation Analysis of Lipids and Glucose Biomarkers in an Elderly Cohort Study

INTRODUCTION

Prior evidence suggested Apolipoprotein E (APOE), lipids, and glucose metabolism may act through the same pathways on the pathogenesis of Alzheimer's disease (AD).

METHODS

This prospective study used data from the Chinese Longitudinal Healthy Longevity Study. We tested the associations of APOE genotype (ε2ε2, ε2ε3, ε2ε4, ε3ε3, ε3ε4, and ε4ε4) and cognitive function using generalized estimating equations (GEE). We examined for possible mediation and effect modification by lipids and glucose level in this association.

RESULTS

APOE ε2 showed significant direct protective effect and indirect harmful effect through TC on cognitive function. Abnormal lipids or glucose levels were not consistently associated with cognitive dysfunction in our study. We did not detect significant indirect effects through lipids for APOE ε4 or any indirect effects through glucose.

DISCUSSION

These findings suggested complicated relationships among APOE, lipids, glucose, and cognitive function. Further study can make validations in other populations.

Liver Bile Acid Changes in Mouse Models of Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disease characterized by progressive cognitive impairment. It is hypothesized to develop due to the dysfunction of two major proteins, amyloid-β (Aβ) and microtubule-associated protein, tau. Evidence supports the involvement of cholesterol changes in both the generation and deposition of Aβ. This study was performed to better understand the role of liver cholesterol and bile acid metabolism in the pathophysiology of AD. We used male and female wild-type control (C57BL/6J) mice to compare to two well-characterized amyloidosis models of AD, APP/PS1, and App^NL-G-F. Both conjugated and unconjugated primary and secondary bile acids were quantified using UPLC-MS/MS from livers of control and AD mice. We also measured cholesterol and its metabolites and identified changes in levels of proteins associated with bile acid synthesis and signaling. We observed sex differences in liver cholesterol levels accompanied by differences in levels of synthesis intermediates and conjugated and unconjugated liver primary bile acids in both APP/PS1 and App^NL-G-F mice when compared to controls. Our data revealed fundamental deficiencies in cholesterol metabolism and bile acid synthesis in the livers of two different AD mouse lines. These findings strengthen the involvement of liver metabolism in the pathophysiology of AD.

A description of the relationship in healthy longevity and aging-related disease: from gene to protein

Human longevity is a complex phenotype influenced by both genetic and environmental factors. It is also known to be associated with various types of age-related diseases, such as Alzheimer's disease (AD) and cardiovascular disease (CVD). The central dogma of molecular biology demonstrates the conversion of DNA to RNA to the encoded protein. These proteins interact to form complex cell signaling pathways, which perform various biological functions. With prolonged exposure to the environment, the in vivo homeostasis adapts to the changes, and finally, humans adopt the phenotype of longevity or aging-related diseases. In this review, we focus on two different states: longevity and aging-related diseases, including CVD and AD, to discuss the relationship between genetic characteristics, including gene variation, the level of gene expression, regulation of gene expression, the level of protein expression, both genetic and environmental influences and homeostasis based on these phenotypes shown in organisms.